Process of electrolytically separating metallic chromium for the production of chromium coatings on other metals



Patented May 21, 1929.

UNITED STATES PATENT OFFICE.

RUDOLF APPEL, 0F BERLIN, GERMANY.

PROCESS OF ELECTROLYTICALLY SEPARATING METALLIC GHROMIUM FOR- THE PRO-DUCTION OF CHROMIUM COATINGS ON OTHER METALS.

No Drawing.

In my prior application for U. S. patent Ser. N 0. 112,408 filed May28th, 1926, I have described a process for the electrolytic separationof metallic chromium for the purpose of coating other metals withmetallic chromium. The said process substantially con-' sists insubjecting the objects to be coated with chromium to electrolysis ascathodes in a bath containing chromic acid to which small quantities ofcertain electrolytes, especially neutral or acidsalts of organic orinorganic acids, have been added. In certain cases the bath isa'cidulated, preferably with sulphuric acid, and it is preferred to useanodes consisting of rolled lead. By this process coatings are obtainedwhich are perfectly non-porous and strongly adherent to the metal to becoated, and the coatings can be obtained according to the conditions invarious colours and, if desired, with polished surfaces without anysubsequent treatment.

Now, on further investigation I have found that especially favourableresults are obtained by using as the addition to the chromic acid bath asalt of a certain acid not mentioned in my prior application, and whileit is not desired to make in the prcsent application any claim for thegeneral idea of making to the bath additions of the eneral naturereferred to in my prior app ication, it is desired to claim in thepresent application the special addition hereinafter referred to.

Therefore, according to the present invention the process forelectrolytically depositing chromium on other metals for producing achromium coating on such metals consists in subjecting the objects. tobe coated as cathodes to electrolysis in a bath consisting of a chromicacid solution to which an alkali metal iodate such as sodium-iodate hasbeen added, in an amount not exceeding .05 gram per litre of bathliquid.

The addition of sulphuric acid is also useful in certain cases wherechromic acid is used, as there are in commerce certain qualities ofchromic acid which are free from sulphuric acid or only contain a smallpercentage thereof.

As the anode I prefer to use electrodes of rolled lead which is moredense than cast lead certain cases may even'be less.

Application filed August 3, 1926, Serial No. 126,928.

and less liable to be attacked by the electro lytic bath and tocontaminate the latter.

The advantage of adding sodium iodate instead of the salts referred toin my said,

Example. To an aqueous solution of chromic acid containing about 5% ofC10,, sodium iodate NaIO, is added in which in most of the cases shoulbe only about 0.5 grams per litre of the bath and in The object to becoated is introduced into the bath as the cathode, and as the anode anelectrode of rolled lead is used.

Electrolysis is carried on at a temperature of from 35 to 40 C. at abath voltage of from 3 to 4 volts and a current density of about .06amperes per square centimetre. The correct acidity of the bath ispreferably ascertained by making a preliminary test, and if in such testthere is a deposit of brown colored oxides on the cathode, somesulphuric acid should be added to the bath to obtain a good metaldeposit.

Under the conditions indicated a pure metallic bright deposit ofchromium is obtained. By varying the temperature, voltage .and currentdensity it is possible to modify the colour of the deposit. Forinstance, at a temperature of about from 10 to 25 C. a brownish blackcoating is obtained.

. What I claim is:

1. A process for depositing chromium on other metals which comprisessubjecting the metallic object to be coated to electrolysis in a chromicacid bath containing not more than 9.5 g'am of sodium iodate per litreof bath 2. A process for depositing chromium on other metals whichcomprises subjecting the more closelyuantitiesmetallic object to becoated to electrolysis in a chromic acid bath containing 0.5 gram ofsodium iodate per litre of bath liquid.

3. A process for depositing chromium on other metals which comprisessubjecting the metallic object to be coated to electrolysis in a chromicacid bath containin about 5% of C10, and 0.5 gram of sodium iodate perlitre of bath liquid, the electrolysis being carried on at a temperatureof from 35 to 40 C. at.

a bath volta'ge of about from 3 to 4 volts and at a current density ofabout .06 amperes per square centimeter.

4; A process of electrolytically separating metallic chromium for theproduction of chromium coatings on other metals, which process consistsin subjecting the objects to be coated as cathodes to electrolysis in achromic acid bath to which an alkali metal 10 iodate has been added in aquantity of 0.5 gram per liter of solution.

In testimony whereof I have affixed my signature.

RUDOLF APPEL.

